1. Field of the Invention
This invention relates to a clutch cover assembly used in an automotive vehicle or other vehicle, and more particularly, relates to a clutch cover assembly equipped with a diaphragm spring.
2. Description of the Prior Art
A conventional clutch cover assembly is shown in FIG. 9. The clutch cover assembly is disclosed in Japanese Utility Model Laid-Open Print No. 64-24732/1989. FIG. 9 is a sectional view of an upper half portion of the clutch cover assembly.
The clutch cover assembly 10 includes a clutch cover 12. The clutch cover 12 is rotatably connected to a flywheel 14 of an engine side at an external portion 12a of plurality the clutch cover 12 through a plurality of equally pitched pins (not shown). A diaphragm spring 16 is disposed in the clutch cover 12 and is supported by a plurality of equally pitched rivets 18 and a plurality of pivot rings 20. Each rivet 18 is fixedly connected to an internal portion 12b of the clutch cover 12 and is inserted into a cutting portion 16a of the diaphragm spring 16. Therefore, the diaphragm spring 16 is prevented from moving with respect to the clutch cover 12 in the circumferential direction and in the vertical direction. The diaphragm spring 16 is held by three pivot rings 20 which are located between an inner wall 12c of the clutch cover 12 and a flange 18a of the rivet 18 so as to be prevented from moving with respect to the clutch cover 12 in the axial direction. The external portion 16b of the diaphragm spring 16 is elastically attached to a pressure plate 22 connected to the clutch cover 12 through a strap (not shown) as an elastic member. As a result, the pressure plate 22 is biased toward the flywheel 14. Therefore, a facing 24 is held by the flywheel 14 and the pressure plate 22 and comes into frictional engagement therewith. The facing 24 is connected to a well-known clutch disk 26 connected to an output shaft of a transmission side. Consequently, a driving force produced by the engine is transmitted from the flywheel 14 to the output shaft. A release bearing 28 is provided at the internal portion 16c of the diaphragm spring 16 and biases the diaphragm spring 16 in a direction of the flywheel 14 when a clutch pedal (not shown) is depressed by the vehicle operator. As a result, the external portion 16b of the diaphragm spring 16 is moved toward the clutch cover 12, because a contacting point between the diaphragm spring 16 and the pivot rings 20 works as a fulcrum point. Thus, the biasing force of the diaphragm spring 16 acting to the pressure plate 22 is decreased according to the movement of the external portion 16b relative to the clutch cover 12. The pressure plate 22 is biased by the strap and is travelled in a direction of the clutch cover 12. Thereby, the engagement of the facing 24 is released. The transmission of the driving force between the flywheel 14 and the output shaft is interrupted.
FIG. 10 is a graph showing the load characteristic of the diaphragm spring 16 and a return spring 30 as described later. The diaphragm spring 16 has the load characteristic curve A and is set at a position or a deflection P in the assembling of the clutch cover assembly. M indicates a set load in accordance with the position P. The right side of the position P indicates a travel of the pressure plate 22 relative to the clutch cover 12. Consequently, when the release bearing 28 biases the diaphragm spring 16 toward the flywheel 14, the external portion 16b of the diaphragm spring 16 is moved toward the clutch cover 12. Therefore, the pressure plate 22 is movable in the range of the right side of the position P. As a result, the load of the diaphragm spring 16 is changeable in response to the travel of the pressure plate 22 in the range of the right side of the position P. The left side of the position P indicates a travel of the pressure plate 22 relative to the flywheel 14. Under the normal condition of the clutch cover assembly 10, the pressure plate 22 fails to move toward the flywheel 14, because the pressure plate 22 is in engagement with the facing 24 as shown in FIG. 9, whereas, as the facing 24 wears by using the clutch cover assembly 10 for a long time period, the pressure plate 22 is moved toward the flywheel 14 by the biasing force of the diaphragm spring 16. The external portion 16b of the diaphragm spring 16 is therefore moved toward the flywheel 14. Thereby, the load of the diaphragm spring 16 is changeable in response to the travel of the pressure plate 22 in the range of the right side of the position P. As a result, when the facings 24 wear, the load of the diaphragm spring 16 becomes greater than the set load M. This causes an increase in the biasing force of the release bearing 28, because when the external portion 16b of the diaphragm spring 16 biases the pressure plate 22 toward the flywheel 14, the internal portion 16c of the diaphragm spring 16 biases the release bearing 28 during the engagement of the facing 24. Consequently, the depression of the clutch pedal is increased in comparison to the normal condition of the clutch cover assembly 10.
In the light of the above-mentioned drawback, the foregoing clutch cover assembly 10 is provided with a return spring 30 which is arranged between the diaphragm spring 16 and the pressure plate 22 as illustrated in FIG. 9. The return spring 30 is supported by the rivets 18 inserted into a hole 30a so as to be prevented from moving with respect to the clutch cover 12 in the circumferential and in the vertical direction. The return spring 30 is held by the pivot rings 20 which are located at both sides thereof so as not to be movable in the axial direction. The external portion 30b of the return spring 30 is bent toward the diaphragm spring 16 and is attached to the surface of the diaphragm spring 16. The return spring 30 also has a load characteristic curve B as shown in FIG. 10. Namely, when the external portion 16b of the diaphragm spring 16 moves toward the flywheel 14 as the facings 24 wear, the return spring 30 biases the diaphragm spring 16 in a direction of the clutch cover 12 so as to cancel the biasing force of the diaphragm spring 16 to the flywheel 14. Accordingly, the load characteristic of the diaphragm spring 16 can be controlled according to the dotted line C. Therefore, the load of the diaphragm spring 16 is less than the set load M even though the facing 24 wears. Thus, the increasing of the depression of the clutch pedal can be prevented.
However, the foregoing clutch cover assembly has the following three problems (a), (b) and (c).
(a) Three pivot rings 20 are required to be held between the diaphragm spring 16 and the return spring 30. As a result, the clutch cover assembly 10 becomes large in the axial direction.
(b) The return spring 30 is firmly held at a location near the hole 30a by the pivot rings 20. Therefore, the stress is centered at the periphery of the hole 30a with the result that the return spring 30 cracks start therefrom.
(c) The return spring is attached to the diaphragm spring 16 at the external portion 16b thereof. Therefore, a span of the return spring 30 which is defined by a distance between the diaphragm spring 16 and the pivot rings 20 is small, because there is small space defined between the pivot rings 20 and the clutch cover 12 in the upward direction. Thereby, the load characteristic of the return spring 30 becomes a sharp curve and a narrow range. Thus, it is difficult to set the position P. Moreover, since the external portion 30b is bent toward the clutch cover 12 in order to have a large span, it brings the complex design of the return spring 30.